As sediment is buried in the earth, the stresses acting on that sediment increase because of the increasing weight of overlying sediments and the effect of tectonic forces. Such changes in stress lead to changes in the properties of the sediment, such as its porosity, permeability, elastic wave velocity, rock strength, and can impact seismic reservoir characterization and well-log measurements. The increase in stress and decrease in permeability with depth can result in buried sediments becoming overpressured if the rate of sedimentation exceeds the rate at which fluid can be expelled from the pore space or if dewatering is inhibited by the formation of seals during burial.
As an example, Figure 1 shows upscaled compressional-wave (P-wave) velocity as a function of depth for a Gulf of Mexico well. The compressional-wave velocity increases strongly with increasing depth to a depth of 12,000 ft (3660 m) as a result of compaction caused by the increasing stress acting on the sediment. Below approximately 12,000 ft (3660 m), however, a trend of decreasing velocity is observed because of an increase in pore pressure below that depth. Overpressured sediments are encountered worldwide, and they often result in drilling problems such as kicks, blowouts, borehole instability, stuck pipe, and lost circulation.
Stresses within the earth also can lead to formation of fractures and faults. Such features depend in part on the geomechanical properties of the sediment, such as rock strength. Stresses also influence the formation of geologic structures such as salt domes and folds.
Figures & Tables
This is a recording of the 2010 Distinguished Instructor Short Course (DISC) by Colin Sayers. The state of stress within the earth has a profound effect on the propagation of seismic and borehole acoustic waves, this leads to many important applications of elastic waves for solving problems in petroleum geomechanics. This course provides an overview of the sensitivity of elastic waves in the earth to the insitu stress, pore pressure, and anisotropy of the rock fabric resulting from the depositional and stress history of the rock, and introduces some of the applications of this sensitivity. The course will provide the basis for applying geophysics and rock physics solutions to geomechanical challenges in exploration, drilling, and production. See catalog #233A for the accompanying DISC book.